A child theme in WordPress is a sub theme that inherits all the functionality, features, and style of its parent theme. Child themes are a safe way to modify a WordPress theme without actually making any changes to the parent theme’s files. When the parent theme gets updated, changes made in the child theme are preserved and applied on the updated version as well. This is why child themes are the safest and best way to make changes to an existing theme. Rather than modifying theme files directly you can simply override them with the templates in the child theme.
In order to make a child theme you must create a folder in your themes directory for your new theme. In this folder the only file you need is style.css. In the header of the style.css file you can specify the parent theme by adding a template line into the comment code where the theme name is written. Because this style sheet is included after the style sheet of the parent it will override any styles in the parent theme’s style.css file.
In order to apply the modifications the child theme has to be activated. The parent theme will still include any functionality that hasn’t been overwritten by the child theme.